Introductory Engineering Graphics

Edward E. Osakue

• Publisher: Momentum Press
• Year: 2018
• ISBN:
  9781947083615
• Topic: Technology & Engineering
• Subtopic: Technology & Engineering Research & Skills
• Index: Technology & Engineering

Introductory Engineering Graphics concentrates on the main concepts and principles of technical graphics.

The chapters and topics are organized in a sequence that makes learning a gradual transition from one level to another. However, each chapter is presented in a self-contained manner and may be studied separately. Chapter 1 discusses guidelines for drafting and Chapter 2 presents the principles and techniques for creating standard multiview drawings. Chapter 3 discusses auxiliary view creation, whereas Chapter 4 focuses on section view creation. Basic dimensioning is covered in Chapter 5. Isometric pictorials are presented in Chapter 6. Working drawings are covered in Chapter 7 and the Appendices provide introductory discussions about screw fasteners, general and geometric tolerancing, and surface quality and symbols.

The book is designed as a material for instruction and study for students and instructors of engineering, engineering technology, and design technology. It should be useful to technical consultants, design project managers, CDD managers, design supervisors, design engineers, and everyone interested in learning the fundamentals of design drafting. The book is in accord with current standards of American National Standards Institute/American Society for Mechanical Engineers (ANSI/ASME). Its principal goal is meeting the needs of first- and second-year students in engineering, engineering technology, design technology, and related disciplines.

CHAPTER 1

GUIDELINES FOR DRAFTING

1.1 INTRODUCTION

Drafting is the process of creating technical drawings consisting of two-dimensional (2D) images and annotations, and the term draughting is used to describe the language of drafting in this book. Draughting defines the terminology, symbology, conventions, and standards used in drafting. It is the universal technical language that is used for clearly and accurately describing the form, size, finish, and color of a graphic design model for construction or recording. Draughting guidelines deal with standards and conventions in drawing media, lettering, linestyes, projection standards, plot scales, dimensioning rules, sectioning rules, and so on. In this chapter, we will concentrate mainly on drawing media, lettering, and linestyles, while others will be discussed in the appropriate chapters.

The 2D images in drafting are constructed from lines and curves, while annotations are composed from characters. 2D technical drawings may be created using axonometric and perspective principles. Axonometric drawings are 2D drawings obtained by applying orthogonal projection principles to three-dimensional (3D) objects and include orthographic, isometric, dimetric, and trimetric drawings. Pictorial drawings such as isometric and perspective drawings mimic 3D objects in appearance, but are made of 2D entities by composition. Most technical drawings are of the orthographic and isometric types, which are the focus of this book. Some standards and conventions apply to both lines and characters in drafting, and they must be learned and used correctly. Therefore, drafting skills involve learning to correctly apply the rules of draughting in creating acceptable or industry standard technical drawings. Proficiency in drafting involves being able to create high-quality technical drawings, therefore, becoming proficient in drafting must be a commitment executed with determined effort.

1.2 CONVENTIONS AND STANDARDS

Draughting principles, conventions, rules, and standards help to minimize misinterpretations of drawing contents and eliminate errors in the communication of technical ideas. Conventions are commonly accepted practices, methods, or rules used in technical drawings. Standards are sets of rules established through voluntary agreements that govern the representation of technical drawings. Standards ensure clear communication of technical ideas. The design drafter must study and understand these conventions and standards and learn to apply them correctly in practice. For example, good technical drawings are achieved by following some principles such as:

1. Keeping all lines black, crisp, and consistent.

2. Using different linestyles.

3. Ensuring clarity in linestyle differences such as in thickness or line weight.

4. Ensuring dashes have consistent spacing with definite endpoints.

5. Keeping guide or construction lines very thin.

6. Ensuring that corners are sharp and without overlap in drawing views.

7. Placing dimension with thoughtfulness and adequate spacing.

8. Making notes simple and concise.

9. Making drawing readability a high priority.

10. Ensuring a pleasing drawing layout.

Principles one to six are largely built into computer design drafting (CDD) software or packages. This means the CDD operator need not worry about them, except know what linestyle to use for different features of objects and assign appropriate line weight or thickness. However, principles 7 to 10 must be mastered and consistently applied. These have bearings on accuracy, legibility, neatness, and visual pleasantness of drawings.

There are national and international organizations that develop and manage the development of standards. Examples are the American National Standards Institute (ANSI) and the International Standardization Organization (ISO). ANSI is a federation of government, private companies, professional, technical, trade, labor, and consumer organizations that serve as a clearinghouse for nationally coordinated voluntary standards. The standards may deal with dimensions, rating, test methods, safety and performance specifications for equipment, products and components, symbols and terminology, and so on. Major contributors to ANSI standards include American Society of Mechanical Engineers (ASME), Institute of Electrical and Electronic Engineers (IEEE), American Society for Testing Metals (ASTM), and so on. Drafting standards are specified in ANSI Y14 documents, which give only the character of the graphic language. It is to contain 27 or more separate sections when completed. ANSI/ASME Y14.2, Y14.3, and Y14.5M are popular draughting standards in the United States and sample sections of the standard are given Table 1.1.

Table 1.1. Some ANSI/ASME Y14 standards

Item	Section
Size and format	Y14.1
Lettering and linestyles	Y14.2
Projections	Y14.3
Pictorial drawings	Y14.4
Dimensioning and tolerancing	Y14.5M
Screw threads	Y14.6
Gears, splines, and serrations	Y14.7
Mechanical assemblies	Y14.14

ISO is a nongovernmental worldwide body that coordinates standards development process in virtually every area of human activities. It is located in Switzerland and was founded in 1947. Membership includes over 150 countries, with each country represented by one national standards institution. ANSI is the U.S. representative to ISO. ANSI standards are usually similar but not identical to ISO standards. The design drafter must be diligent in adhering to the standards that are relevant to a particular work. Table 1.2 gives some ISO drawing standards documents.

Table 1.2. Some ISO drawing standards

Item	Section
Technical drawings: sizes and layout of drawing sheets	ISO 5457
Technical drawings: general principles of presentation	ISO 128
Technical drawings: methods of indicating surface texture	ISO 1302
General tolerances	ISO 2768

1.3 DRAWING UNITS

All engineering drawings must carry a unit of measure. This is required so that the drawing sizes can be correctly interpreted. Because graphics have linear and angular attributes, the units of length and angles are indispensable in drafting and design.

1.3.1 UNITS OF LENGTH

The SI unit of length is the meter. The English or U.S. customary unit of length is the foot (ft). Table 1.3 shows the length denominations for SI and English units. English units are still in use in North America, especially in the United States.

The SI linear unit for drafting is the millimeter. Mechanical drawings are dimensioned in millimeter (mm). Architectural drawings may be dimensioned in millimeter (mm) and meter (m). Meter and kilometer (km) are used for civil dimensioning. Only decimals are used in metric dimensioning; fractions are not allowed. For numbers less than 1.0, which must be expressed as decimals, a zero before the decimal marker is preferred. For example, 0.234 is preferred to .234. The period symbol is the decimal marker in this example. In Europe and some other countries, “,” is used as decimal marker, i.e. 0,234 means the same as 0.234 in North America.

In English units, mechanical drawings are dimensioned in decimal inches, architectural drawings are commonly dimensioned in feet (‘), and fractional inches and civil drawings are dimensioned in decimal feet and inches. In North America, drawings in metric units carry a general note such as “all dimensions are in millimeter, unless otherwise stated” or the label “METRIC.”

Table 1.3. Drawing units

SI: meter (m)	Customary: Inch (in)-foot (ft)
1 m = 1,000 mm = 10^3 mm	1 in = 16 lines
1 m = 100 cm = 10^2 cm	1 ft = 12 inches
1 km =...